1. Field of the Invention
The present invention relates to metallic foil useful as a hydrogen-permeable film (membrane) in a hydrogen-refining unit used in fields related to fuel cells and semiconductors.
2. Background Art
In recent years, as a countermeasure against global warming, there is demand for practical use and spread of a hydrogen-refining unit and fuel cells utilizing the same. Such a hydrogen-refining unit includes first and second chambers, and the first chamber is separated via a membrane from the second chamber. When a hydrogen-containing gas is passed through the first chamber, the membrane permits hydrogen to be substantially permeated therethrough, whereby a hydrogen-enriched gas is collected in the second chamber while a gas containing impurities (CO, CO2 etc.) remains in the first chamber. As described above, the membrane in the hydrogen-refining unit should be hydrogen-permeable.
As such membrane, palladium alloy (e.g. Pdxe2x80x94Cu) foil capable of occluding hydrogen has been used. The palladium alloy foil has excellent hydrogen permeability, but because palladium is relatively expensive, there is a need for a substitute made of a cheaper material than the palladium alloy foil.
As a substitute for the palladium alloy, a vanadium alloy (Vxe2x80x94Nixe2x80x94Ti, Vxe2x80x94Nixe2x80x94Zr) has been examined. However, the vanadium alloy is poor in rolling ability so that when vanadium alloy foil is produced by rolling molding, special rolling conditions and repetition of an annealing step are necessary, thus increasing production costs. Further, if annealing is repeated in producing the foil, element distribution in the foil may be segregated. Moreover, such procedures should be conducted in an inert gas atmosphere in order to prevent oxidation of the vanadium alloy, but the facilities should be large-scaled when conducting the rolling and annealing steps in an inert-gas atmosphere. In addition, the vanadium alloy foil produced by rolling molding is low in toughness and poor in processability and durability.
The object of the present invention is to provide a process for producing vanadium alloy foil excellent in hydrogen permeability and processability, free from segregation of element distribution in the foil, and useful as a membrane in a hydrogen-refining unit.
To solve the problem described above, the inventors of the present application made extensive study, and as a result, they found that the problem can be solved by a process for producing foil of a vanadium alloy comprising 5 to 25% by weight of at least one selected from the group consisting of Ni, Co, Mo, Fe and Ag, and 0.01 to 5% by weight of at least one selected from the group consisting of Ti, Zr and Y, the balance being V, which comprises the steps of:
preparing a melt of the vanadium alloy by use of a crucible having a slit in the bottom,
rotating a roll comprising a cylinder whose central axis is arranged to be parallel to the slit,
jetting the melt from the slit to the roll surface of the rotating roll and rapidly cooling the melt jetted from the slit, and
continuously exfoliating the vanadium alloy solidified on the roll surface from the roll surface to obtain the foil.